1. The effects of exogenous corticosterone on plasma corticosterone concentrations, body weight, food intake, the percentage of birds that laid an egg each day and egg weight were measured in laying Japanese quail. Data were collected from birds in the week before corticosterone treatment began, during a 21-d treatment period, and in the 22 d after treatment ended. 2. Groups of quail were treated with corticosterone at three different concentrations in their drinking water. Corticosterone intakes were calculated for each bird and birds were then assigned to 4 intake groups. The groups were 0 (control group), 0.31-0.60, 0.61-0.90, 0.91-1.50 or >1.51 mg corticosterone/bird/d. 3. Plasma corticosterone concentrations in the 4 intake groups increased from around 1 ng/ml on the day before corticosterone treatment began to maximum mean concentrations of 13-18 ng/ml on day 14 of treatment. Mean corticosterone concentrations did not change between day 14 of treatment and the day after treatment ended, and had decreased in only one of 4 intake groups one week later. 4. Mean body weight in the highest intake group remained significantly lower than in controls 22 d after corticosterone treatment ended. Whilst there was no clear effect of corticosterone on food intake during treatment, mean food intake in the three highest corticosterone intake groups was significantly lower than in controls in the week after treatment ended. The percentage of birds that laid an egg each day and egg weight were both decreased by corticosterone, and the percentage of birds that laid an egg each day remained significantly lower in the highest corticosterone intake group compared with controls in the third week after treatment ended. 5. It is suggested that elevated plasma corticosterone concentrations in quail after treatment ended were maintained by a hyperactive hypothalamo-pituitary-adrenal axis for at least one week. Although none of the negative effects of corticosterone were evident in the group of quail with the lowest corticosterone intakes, the findings of the present study show that corticosterone treatment can affect birds for up to several weeks after corticosterone intake ends.